Some circuits have lower voltage portions that operate at a lower supply voltage and higher voltage portions that operate at a higher supply voltage. Level-shifters convert a lower voltage control signal received from a lower voltage portion (hereinafter, a “controlling circuit”) of the circuit into a higher voltage control signal suitable for controlling a higher voltage portion (hereinafter, a “controlled circuit”) of the circuit.
In some cases, the controlled circuit might have a varying voltage supply. Sometimes, the controlled circuit might operate with the high voltage supply as explained above (hereinafter called the “high voltage condition” of the controlled circuit). But other times, the controlled circuit might operate with a lower voltage supply, perhaps even as low as, or even lower than, the voltage supply of the controlling circuit (hereinafter called the “low voltage condition” of the controlled circuit). For instance, an Electrically Erasable and Programmable Read-Only Memory (EEPROM) might sometimes operate in a low voltage condition during a read operation, but might operate at a high voltage condition during a program operation.
In order to support this switching operation, the level-shifter and the controlled circuit often begins the programming operation in a low voltage condition. Then, the supply voltage to the level-shifter is raised causing the controlled circuit to transition to the high voltage condition. The supply voltage is then lowered again causing the controlled circuit to return to the low voltage condition.
For proper operation, the output of the level-shifter should be responsive to the input signal of the level-shifter, even at the low voltage condition. In other words, the output signal should switch to the state corresponding to the input signal state, even if the level-shifter has a lower voltage supply.